92 PRACTICAL ANATOMY. 



Each vertebra develops from three primary centres, 

 which appear about the middle of the second month. 

 The primary centres are one for the body and one for 

 each lateral arch. Besides these, six secondary centres 

 appear later in life, completing the transverse and spinous 

 processes, and in the articular plates on the superior and 

 inferior surfaces of the body. 



Exceptions. The atlas develops by three centres, 

 one for the anterior arch and one for each lateral mass. 

 The axis develops by six centres,^one for the body and 

 one for each lateral arch, and three for the odontoid 

 process. The lumbar vertebrae have, in addition to the 

 number for the other vertebrae, a centre for each tubercle 

 behind the superior articular surface. 



The spine, viewed laterally and in profile, presents 

 two anterior convex and two anterior concave curves, 

 the former in the cervical and lumbar, and the latter in 

 the dorsal and sacral regions. These curves are largely 

 due to the form of the bodies of the vertebrae, which, in 

 the cervical region, are thicker anteriorly than posteriorly ; 

 in the dorsal region they are thicker posteriorly, and in 

 the lumbar, especially the fifth lumbar, thicker anteriorly 

 than posteriorly, as in the cervical region. The spinal 

 canal is largest in the cervical, next in the lumbar, and 

 smallest in the dorsal regipn. There is a limited motion 

 in every intervertebral articulation, but, by the combined 

 movement of the vertebrae upon one another, a great 

 variety of movements, such as bending and lateral 

 rotation of the trunk and head, are possible. The articu- 

 lations between the cervical vertebrae admit of the great- 

 est motion, next in the lumbar, and least in the dorsal. 

 The direction of the spines of the dorsal vertebrae down- 

 ward locks them together, and the position and plane of 

 the articular surfaces also serve to give rigidity to this 



